The present invention relates to a positive electrode material for alkaline storage batteries, more particularly to an active material for a non-sintered type positive electrode for alkaline storage batteries, and a method of producing the same.
With the recent popularization of portable equipments, realization of high capacity has been required strongly to an alkaline storage battery. Particularly, a nickel-metal hydride storage battery, which is a secondary battery comprising a positive electrode mainly using nickel hydroxide as an active material and a negative electrode using a hydrogen storage alloy as a main material, has rapidly been popularized as a secondary battery having high capacity and high reliability.
The positive electrode for alkaline storage batteries will be described hereinafter.
The positive electrode for alkaline storage batteries is roughly classified into two types, e.g. sintered type and non-sintered type. The former is produced by impregnating a porous nickel sintered plaque having a porosity of about 80%, obtained by sintering a core material such as a perforated metal and a nickel powder, with an aqueous solution of a nickel salt such as nickel nitrate, followed by impregnating with an alkaline solution, thereby to form nickel hydroxide in the porous nickel sintered plaque. Regarding this positive electrode, it is difficult to make the porosity of the plaque larger than that of a conventional positive electrode and, therefore, an amount of the active material to be filled can not be increased, which results in limit of high capacity.
The latter non-sintered type positive electrode is produced, for example, by filling a three-dimensionally continuous porous foamed nickel substrate having a porosity of not less than 95% with nickel hydroxide particles as disclosed in Japanese Laid-Open Patent Sho 50-36935, and is widely used as a positive electrode for high-capacity alkaline storage battery.
As an active material for the non-sintered type positive electrode, spherical nickel hydroxide particles having high bulk density and filled in a foamed substrate are used in view of realization of high capacity. As the nickel hydroxide particles, metallic ions of cobalt, cadmium, zinc, or the like are generally incorporated in part with the nickel hydroxide particles in view of the improvement of discharge characteristic, charge acceptance and cycle life. Regarding the porous foamed substrate used in the non-sintered type positive electrode, the pore size is from about 200 to 500 .mu.m and these pores are filled with spherical nickel hydroxide particles having a particle diameter of several .mu.m to several tens .mu.m. The charge/discharge reaction of the nickel hydroxide particles, which are present in the vicinity of a skeleton of the substrate where current collection is satisfactorily performed, proceeds smoothly, but the reaction of the nickel hydroxide particles which are present apart from the skeleton does not proceed sufficiently.
Accordingly, in the non-sintered positive electrode, means for electrically connecting nickel hydroxide particles by using a conductive agent are employed so as to improve the utilization of the filled nickel hydroxide particles. As the conductive agent, a single metal such as cobalt, nickel, etc. is used sometimes, but a divalent cobalt oxide such as cobalt hydroxide, cobalt monoxide, etc. is used in many cases. These divalent cobalt oxides have no conductivity, intrinsically. It is considered, however, these divalent cobalt oxides are electrochemically oxidized by initial charging in the battery to be converted into P-cobalt oxyhydroxide having the electric conductivity, which effectively serves as a conductive network. By the presence of this conductive network, it becomes possible to greatly enhance the utilization of nickel hydroxide particles filled in high density in the non-sintered type positive electrode, thereby to realize high capacity compared with the sintered type positive electrode.
However, even in the non-sintered positive electrode having the above described construction, the conductive performance of the network is not perfect and there is a limit in utilization of the nickel hydroxide particles. Furthermore, the above positive electrode has a drawback that, when a battery is overdischarged or allowed to stand in the state of short circuit, or stored for a long period of time or stored at high temperatures, the positive electrode capacity is reduced.
This is because divalent cobalt oxide cannot be completely converted into .beta.-cobalt oxyhydroxide by the above described electrochemical oxidation reaction.
Recently, as means for improving the imperfection of the above positive electrode conductive network, Japanese Laid-Open Patent Hei 8-148145 (or U.S. Pat. No. 5,629,111) discloses a technique of heating (oxidizing) a cobalt hydroxide in a positive electrode material in the presence of an aqueous alkaline solution and oxygen (air) outside the battery, thereby to convert into a cobalt oxide having a distorted crystal structure and a cobalt valence higher than 2. The patent publication discloses oxidation to a cobalt oxide having a valence of about 2.9 and characteristics of a battery using the same.
The above publication further discloses that nickel hydroxide particles having a coating layer of a cobalt hydroxide (hereinafter referred to as Co(OH).sub.2 -coated nickel hydroxide particles) are subjected to the above heating treatment. The Co(OH).sub.8 -coated nickel hydroxide particles are prepared by using a method of stirring nickel hydroxide particles in an aqueous solution of a divalent cobalt salt and adjusting the pH while adding dropwise an aqueous alkaline solution, thereby to deposit a cobalt hydroxide on the surface of the particles (liquid phase method), or a method of adding a cobalt hydroxide powder to nickel hydroxide particles and coating the surface of the nickel hydroxide particles with the cobalt hydroxide utilizing an action of a shear force or an impact force due to mechanical mixing (mechanical mixing method), and have been known for a long time as means for enhancing the dispersibility of cobalt in the positive electrode.
When the Co(OH).sub.2 -coated nickel hydroxide particles are subjected to the above heat-treatment, there can be obtained a positive electrode material capable of providing a considerably good conductive network by the combination of high dispersibility of the cobalt and bonding of the interface between the nickel hydroxide mother particles and the coating layer as the oxidation of cobalt proceeds even if the amount of cobalt used is small.
As the method for production of the above positive electrode material, Japanese Laid-Open Patent Hei 9-73900 discloses a method of flowing Co(OH).sub.2 -coated nickel hydroxide particles containing an aqueous alkaline solution in a batch fluidized drier, or heating the particles with dispersing. According to this method, there is an advantage that agglomeration of particles due to the treatment can be prevented.
However, the oxidation condition of the cobalt hydroxide constituting the coating layer on the surface of the active material particles is not satisfactory in the positive electrode material for alkaline storage batteries disclosed in the above publication and there still exists some room for improvement. This is because proceeding of the oxidation of the cobalt hydroxide in the presence of alkali greatly depends not only on the ambient temperature and the concentration of an aqueous alkaline solution which is present around the cobalt hydroxide, but also on humidity and oxygen concentration of the environment, so that it is impossible to prevent unreacted cobalt hydroxide from remaining and a side reaction from occurring.
Considering this point, the inventors of the present invention have conducted detailed experiments and analyses and suggested that the utilization of a positive electrode material should be improved by oxidizing the cobalt oxide constituting the coating layer to .gamma.-cobalt oxyhydroxide with cobalt valence of larger than 3.0. The .gamma.-cobalt oxyhydroxide contains a large amount of alkali cation (K.sup.+ or Na.sup.+) in the crystal. A positive electrode comprising above-mentioned .gamma.-cobalt oxyhydroxide is disclosed in U.S. patent application Ser. No. 08/991,415 filed Dec. 16, 1997, which is incorporated herein by reference in its entirely.
However, it has turned out that although when the cobalt oxide constituting the coating layer has a cobalt valence larger than 3.0, the characteristics of the positive electrode material are greatly improved; there is a drawback that the capacity is more deteriorated than those of conventional positive electrodes as the charge/discharge cycle is repeated at high temperatures.
That is, when a battery is discharged until the battery voltage reaches around 0.8V at high temperatures, namely 40.degree. C., part of the cobalt oxide constituting the coating layer of the active material particles causes discharge reaction (reduction) at the end of the discharging. The reason for this is that the positive electrode is likely to discharge at high temperatures since the internal resistance of the battery decreases and that the cobalt oxide has extremely high electric conductivity due to a large valence of cobalt. In addition, alkali cation (K.sup.+ or Na.sup.+) contained in the cobalt oxide eliminates although the amount is very small.
When the battery is charged, a charge reaction (oxidation) of the above-mentioned discharged cobalt oxide may occur at the beginning of the charging. However, the capacity to charge the cobalt oxide becomes slightly smaller than the discharged capacity due to the elimination of alkali cation during the discharging and other reasons. That is, when a charge/discharge reaction is conduced at high temperatures, the cobalt valence of the cobalt oxide constituting the coating layer slightly decreases. Then, the charge/discharge cycle is repeated at high temperatures, the above-mentioned phenomena are accumulated, and as a result, the cobalt oxide of the coating layer approaches a thermodynamically stable structure such as CoO(OH) or Co.sub.3 O.sub.4 (both are oxides with poor electric conductivity) while decreasing the cobalt valence. Consequently, the performance of the conductive network of the positive electrode becomes insufficient, which decreases the capacity.